Soft matter with intelligence

We are developing a range of responsive dynamic soft gel-based materials for advanced delivery systems, rheology modifiers, and soft robotics. Recent examples are our hydrogel objects which can show programmed independent responsive behavior and communication, and our development of our moldable responsive HIPE-gel objects and fibers.

The independent action of fibres of different compositions in a closed system. Three populations of fibres can be seen: 10 g/L (red), 8 g/L (blue) and 4 g/L (orange). Fibres of a higher enzyme concentration start to disassemble before those of a lower concentration. At the beginning, all fibres are intact. After 78 seconds, only the red fibres have released their oil droplets. After 614 seconds, the blue fibres have released their droplets and after 1030 seconds, the orange fibres have released theirs. Scale bar = 1 cm.

In our 2017Materials Horizons paper we show the fabrication of soft hydrogel alginate-based objects, namely fibres and beads, that have an individually programmed time delay in their response to a shared environmental stimulus. We utilize the enzyme urease to programme a self-regulated change in pH, which in turn activates the designed response of gel fibre disintegration or a change in gel beads colour. This design allows for independent response behaviour of a collection of bodies in a single closed system, as well as inter-material communication on shorter length scales. The incorporation of responsive time control directly into soft matter objects demonstrates an advance in the field of autonomous materials. You can read the paper here: http://dx.doi.org/10.1039/C7MH00033B

In our 2016 Journal of Materials Chemistry A paper we show that emulsion droplets stabilized by branched copolymers and Laponite clay discs can be assembled into supracolloidal fibers with control of the fiber composition and length. We call these fibers HIPE (high internal phase emulsion) fibers and they are composed of thousands if not millions of emulsion droplets. Upon drying they transform into a light-weight highly porous nanocomposite material. We demonstrate that the fibers made from emulsion droplets can be used to release volatile compounds in a time-controlled manner. More on our blog. You can read the (open access) paper here: http://dx.doi.org/10.1039/C5TA08917D

In our 2013 Chemical Communications paper we report the fabrication of various shaped High Internal Phase Emulsion hydrogels. Key is the use of dilute waterborne poly(N-isopropylacrylamide) microgel dispersions which are non-covalently crosslinked through 2-ureido-4[1H] pyrimidinone (UPy) quadruple hydrogen bond groups. These microgels were developed in one of our earlier papers published in 2013 in Polymer Chemistry (read the paper here: http://dx.doi.org/10.1039/C2PY20615C). The colloidal particles position themselves at the oil-water interface hereby serving as Pickering stabilizers. Over time they deform and with the excess amount in the continuous water phase they form a non-covalently UPy-crosslinked hydrogel monolith. The reversible UPy crosslinks allow for the HIPE-hydrogels to be molded into objects which are thermo-responsive in Nature. You can read the paper here: http://dx.doi.org/10.1039/C2CC38200H

Active Matter: swimmers and shakers

Self-propulsion, (chemo)taxis, swarming, catalytic release

Colloids in motion

We have a strong interest in fabricating colloidal systems in which we can control the motility of the particles when they are dispersed in a liquid. We are designing systems that can swim, shake, and swarm.

We are pleased that our work on using catalytic colloidal particles to control the permeability of vesicles was selected to feature on the Jan-Feb 2016 cover of Materials Horizons. In this work, giant polymer vesicles which have membrane-embedded catalytically active manganese oxide particles are made using droplet-based microfluidics. It is demonstrated that these colloidal particles can regulate the membrane permeability of the polymersomes upon their exposure to, and catalytic reaction with, small amounts of dissolved hydrogen peroxide. Not only can we have triggered complete release whereby the vesicle gets destroyed through membrane rupture by the formed oxygen bubbles as illustrated on the cover, exposure to small amounts of dissolved hydrogen peroxide leads to temporary enhanced release until all hydrogen peroxide is consumed by the catalytic particles after which the membrane permeability restores itself to its passive characteristic value.

Annular Dark Field Scanning TEM analysis of our matchstick particles which have a catalytic manganese oxide enriched head and a tail of silica. In our mechanistic study reported in 2015 in Langmuir we report an insight into the synthesis of silica-based “matchstick”-shaped colloidal particles, which are of interest in the area of self-propulsion on small length scales. The generation of aqueous emulsion droplets dispersed in an n-pentanol-rich continuous phase and their use as reaction centers allows for the fabrication of siliceous microparticles that exhibit anisotropy in both particle morphology, that is, a “matchstick” shape, and chemistry, that is, a transition-metal oxide-enriched head. We provide a series of kinetic studies to gain a mechanistic understanding and unravel the particle formation and growth processes. Additionally, we demonstrate the ability to select the aspect ratio of the “matchstick” particle in a straightforward manner.

In our 2014 Materials Horizons paper which made the inner cover of the first issue of this journal we report the discovery of our matchstick particles and we demonstrate that they can undergo chemotaxis, as they self-propel upwards on a hydrogen peroxide gradient in water. You can read the paper here: http://dx.doi.org/10.1039/C3MH00003F

Particles at Soft Interfaces

Pickering Stabilization

Pickering stabilization - particles can adhere to soft interfaces.

The phenomenon that particles can adhere to soft interfaces is known as Pickering stabilization. Over the last decade we have studied the behavior of particles at liquid-liquid interfaces, and we have used Pickering stabilization to develop the processes of Pickering mini-emulsion polymerization and Pickering emulsion polymerization. We also have used the ability for particles to adhere to interfaces as a tool to make more complex supracolloidal structures, such as Pickering high internal phase emulsion solid polymer and gel-based monoliths.

There has been much scientific interest in the behaviour of colloidal particles at liquid interfaces. From a research aspect they provide model systems for fundamental studies of condensed matter physics. From a commercial aspect they provide applications for making new materials in the cosmetics, food and paint industries.

In many cases of colloidal particles at interfaces, the mechanism of particle interactions is still unknown. Particle-Stabilized Emulsions and Colloids, available from the RSC from January 2015 looks at recent studies on the behaviour of particles at liquid interfaces. The book first introduces the basic concepts and principles of colloidal particles at liquid-liquid interfaces including the interactions and conformations. The book then discusses the latest advances in emulsions and bicontinuous emulsions stabilized by both solid and soft particles and finally the book covers applications in food science and oil extraction.

With contributions from leading experts in these fields, this book will provide a background to academic researchers, engineers, and graduate students in chemistry, physics and materials science. The commercial aspects will also be of interest to those working in the cosmetics, food and oil industry.

This image is taken from one of our latest papers, entitled Equilibrium orientations of non-spherical and chemically anisotropic particles at liquid–liquid interfaces and the effect on emulsion stability, on modelling the behavior of anisotropic Janus particles at oil-water interfaces. The paper was published in March 2015 in the Journal of Colloid and Interface Science. Top: Janus ellipsoids with aspect ratio of 0.4 from left to right Xplane = 0, 0.5, −0.5 in the xy plane and Xplane = 0, 0.5, −0.5 in the xzplane. Middle: escape energy for polystyrene-poly(HEMA) Janus ellipsoids (aspect ratio = 0.4) of varying Janus character either intersected in the xy plane (squares) or the xz plane (triangles). The colour of the point represents that the minimum energy for escape is into the oil phase (red) or aqueous phase (black).

σHD/water = 53.5 mN m−1, σHD/PSt = 14 mN m−1, σwater/PSt = 32 mN m−1,σHD/PHEMA = 18 mN m−1, σwater/PHEMA = 12 mN m−1 either calculated from the polymer surface energy or taken from literature. Bottom: free energy profile for Xplane = 0 in the xy axis (left) and xz axis (right). The grey circles show minima in the free energy profile. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Polymer and Inorganic Colloids

We are experts in heterogeneous polymerization techniques

We love designing complex colloidal particles

We have an in-depth knowledge of heterogeneous polymerization techniques, such as emulsion and mini-emulsion polymerizations, to develop a plethora of complex polymer-based colloids. We recently have expanded our particle synthesis and characterization portfolio to inorganic colloids, such as silicates, metal oxides and calcium carbonate.

We built advanced materials from colloids

We use colloidal particles as building blocks for advanced supracolloidal materials. Our portfolio contains the development of waterborne coatings that have resistance to organic solvents, gas sensors, armored colloidal systems for delivery of drugs or benificial agents, and polymer particles modified hybrid biological plant spores

The vast majority of reports on Janus particles deal with the fabrication and behavior of anisotropic particles that have both lyophilic and lyophobic features. Opposing characteristics on a single particle, however, can span a wide variety of physical properties. We have fabricated dumbbell or peanut-shaped microparticles that have one hard polystyrene lobe, and one soft lobe made from poly(n-butyl acrylate). A mechanistic study on their synthesis we have reported in the ACS Journal Langmuir in 2014 entitled Synthesis of "hard-soft" Janus particles by seeded dispersion polymerization (you can read the paper here: http://dx.doi.org/10.1021/la503366h). We found in a study reported in 2014 in Soft Matter that these "hard-soft" Janus particles upon collision under sheared and dilute condition, whereby physisorbed poly(vinyl pyrrolidone) which served as steric stabilizer desorbed from the particles rendering them colloidally unstable, these could assemble into distinct microscopic supracolloidal analogues of simple molecular valance shell electron pair repulsion (VSEPR) space-fill structures. Simulations of expected cluster morphology, compared with those from cryo-SEM analysis support the mechanism of assembly driven by surface area minimization in the case of soft–soft interactions. Altering the soft lobe size with respect to the hard lobe indicates a moderate effect on number of primary particles per cluster. Additionally, higher order structures of clusters containing a number of primary particles exceeding what is possible for a ‘solid’ core cluster are observed. As such, we also investigated the formation of suprastructures using a high number of ‘hard–soft’ Janus particles and verified their effective Pickering stabilization of air bubbles. You can read the paper here: http://dx.doi.org/10.1039/C4SM01708K.